This invention relates to a process and apparatus for the purification of an organically contaminated liquid in the presence of activated sludge, wherein an oxygen-enriched gas is introduced into the liquid within an aeration tank, and an atmosphere more enriched with oxygen than air is maintained at least above the largest part of the liquid, and wherein a portion of the sludge obtained in a secondary settling tank is recycled as a return sludge into the aeration tank.
Processes for the purification of an organically contaminated liquid (e.g., sewage) are known wherein a gas having a higher concentration of oxygen than air is fed to the liquid in a sealed aeration tank in the presence of activated sludge. A portion of the liquid-sludge mixture produced in the aeration tank is continuously transferred into a secondary settling tank wherein the sludge is separated from the liquid, and at least a portion of the sludge obtained in the secondary settling take is continuously recycled into the aeration tank.
The most serious disadvantage of these conventional methods is that the purification efficiency is not constant per unit time, especially when degrading high concentrations of organic impurities, but rather decreases with time. Within the aeration tank, the liquid phase as well as the gaseous phase, i.e. the atmosphere present in the aeration tank, are constantly enriched with CO.sub.2 as a consequence of the biological reactions effecting the purification of the liquid. This is especially the case with high BOD.sub.5 concentrations in the liquid since large oxygen consumptions are required resulting in extensive metabolic products, particularly CO.sub.2. This CO.sub.2 concentration, which is constantly on the increase in the aeration tank and cannot be reduced by the very low inert gas concentrations in the aeration gas, leads to an extremely disadvantageous effect on oxygen utilization and to a constant reduction in the pH value of the liquid-sludge mixture; as a consequence thereof, the purification efficiency is constantly on the decrease.